Volcano Update from Archive

Thursday, March 19, 2009 19:29 PDT (Friday, March 20, 2009 02:29 UTC)

40.49°N 121.51°W, Summit Elevation 10456 ft (3187 m)
Volcano Alert Level: NORMAL
Aviation Color Code: GREEN

Information Statement for the recent March 17-19, 2009, Lassen Peak swarm activity

A series of small earthquake swarms have occurred beneath the south flank of Lassen Peak in Lassen Volcanic National Park over the last three days. This activity began at 3:00 AM (PDT) on March 17 and persisted through 8:16 AM on the 19th. Episodic swarm activity on the 17th through 4:10 AM on the18th included some 48 earthquakes large enough to be located, the three largest of which had magnitudes of M=2.1, 2.0, and 2.2. Activity resumed at 6:57 AM on the 19th producing 26 earthquakes by 8:19 AM, the largest of which was a M=1.9. These earthquakes are centered at a depth of roughly 5 km beneath the surface and, in map view, form an elongated pattern extending from the vicinity of Eagle Peak on southwest flank of Lassen Peak to the vicinity of Diamond Peak 3 km further to the south-southwest.

Earthquake swarms are not unusual in this area. This most recent activity was preceded by a swarm of 12 earthquakes on February 13-14 that included a M=1.8 event. Earlier activity in this same area over the last five years includes swarms on February 15-16, 2005 (14 events including a M=2.3 earthquake), February 21 2006 (13 events with a M=2.2 earthquake), January 18-19, 2007 (29 events with a M=1.9 earthquake), and September 7-8, 2008 (41 events with a M=2.3 earthquake). This swarm activity over the past 3 days appears to be within the norm of earthquake activity detected in the Lassen area since the modern seismic network was installed in 1980. The USGS will continue to monitor the situation closely.

All other volcanoes in the Cascade Range are at normal levels of background seismicity. These include Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams in Washington State; Mount Hood, Mount Jefferson, Three Sisters, Newberry Volcano, and Crater Lake, in Oregon; and Medicine Lake volcano and Mount Shasta in northern California.

The U.S. Geological Survey and University of Washington continue to monitor these volcanoes closely and will issue additional updates and changes in alert level as warranted.

For additional information, background, images, and other graphics: http://vulcan.wr.usgs.gov
For seismic information on Oregon and Washington volcanoes: http://www.pnsn.org/WEBICORDER/PNSN/welcome.html
For seismic information on California volcanoes: http://earthquakes.usgs.gov/eqcenter/recenteqsus/Maps/special/California_Nevada.php
For a definition of alert levels: http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/volcano_warning_scheme.html
For a webcam view of Mount St. Helens: http://www.fs.fed.us/gpnf/volcanocams/msh/